340 Chugging instead of Reving

Since you didn’t degree the cam, take off the drivers side valve cover. Roll the engine over until you are TDC on over overlap (180 degrees from number 1 firing) and look at your valves.

If both valves are open the same, you are at split overlap and the cam is in straight up, whatever that is.

If the intake valve is open more than the exhaust valve, the cam is advanced from split overlap (straight up). You won’t know exactly how much, but you will know it’s advanced.

If the exhaust valve is open more than the intake valve, the cam is retarded from split overlap (straight up). You won’t know how much, but you will know its retarded.

Like I said, something is weird. I’m thinking cam timing is the first place I’d check.
Rat I agree about the cam timing should have been degreed in.
If the cam timing were way off wouldn't it run poorly through out the entire rpm range? Food for thought.
The plugs a pig rich but that again would not cause a sudden change at 3000 rpm.
Im inclined to point fingers at the ignition system like other members have suggested.